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        <div>
            <div><h3 class="title">
                <a name="asio.reference.execution_context"></a><a class="link" href="execution_context.html"
                                                                  title="execution_context">execution_context</a>
            </h3></div>
        </div>
    </div>
    <p>
        <a class="indexterm" name="asio.indexterm.execution_context"></a>
    </p>
    <p>
        A context for function object execution.
    </p>
    <pre class="programlisting"><span class="keyword">class</span> <span
            class="identifier">execution_context</span> <span class="special">:</span>
  <span class="identifier">noncopyable</span>
</pre>
    <h5>
        <a name="asio.reference.execution_context.h0"></a>
        <span><a name="asio.reference.execution_context.types"></a></span><a class="link"
                                                                             href="execution_context.html#asio.reference.execution_context.types">Types</a>
    </h5>
    <div class="informaltable">
        <table class="table">
            <colgroup>
                <col>
                <col>
            </colgroup>
            <thead>
            <tr>
                <th>
                    <p>
                        Name
                    </p>
                </th>
                <th>
                    <p>
                        Description
                    </p>
                </th>
            </tr>
            </thead>
            <tbody>
            <tr>
                <td>
                    <p>
                        <a class="link" href="execution_context__id.html" title="execution_context::id"><span
                                class="bold"><strong>id</strong></span></a>
                    </p>
                </td>
                <td>
                    <p>
                        Class used to uniquely identify a service.
                    </p>
                </td>
            </tr>
            <tr>
                <td>
                    <p>
                        <a class="link" href="execution_context__service.html" title="execution_context::service"><span
                                class="bold"><strong>service</strong></span></a>
                    </p>
                </td>
                <td>
                    <p>
                        Base class for all io_context services.
                    </p>
                </td>
            </tr>
            <tr>
                <td>
                    <p>
                        <a class="link" href="execution_context/fork_event.html"
                           title="execution_context::fork_event"><span
                                class="bold"><strong>fork_event</strong></span></a>
                    </p>
                </td>
                <td>
                    <p>
                        Fork-related event notifications.
                    </p>
                </td>
            </tr>
            </tbody>
        </table>
    </div>
    <h5>
        <a name="asio.reference.execution_context.h1"></a>
        <span><a name="asio.reference.execution_context.member_functions"></a></span><a class="link"
                                                                                        href="execution_context.html#asio.reference.execution_context.member_functions">Member
        Functions</a>
    </h5>
    <div class="informaltable">
        <table class="table">
            <colgroup>
                <col>
                <col>
            </colgroup>
            <thead>
            <tr>
                <th>
                    <p>
                        Name
                    </p>
                </th>
                <th>
                    <p>
                        Description
                    </p>
                </th>
            </tr>
            </thead>
            <tbody>
            <tr>
                <td>
                    <p>
                        <a class="link" href="execution_context/execution_context.html"
                           title="execution_context::execution_context"><span
                                class="bold"><strong>execution_context</strong></span></a> <span class="silver">[constructor]</span>
                    </p>
                </td>
                <td>
                    <p>
                        Constructor.
                    </p>
                </td>
            </tr>
            <tr>
                <td>
                    <p>
                        <a class="link" href="execution_context/notify_fork.html"
                           title="execution_context::notify_fork"><span class="bold"><strong>notify_fork</strong></span></a>
                    </p>
                </td>
                <td>
                    <p>
                        Notify the execution_context of a fork-related event.
                    </p>
                </td>
            </tr>
            <tr>
                <td>
                    <p>
                        <a class="link" href="execution_context/_execution_context.html"
                           title="execution_context::~execution_context"><span
                                class="bold"><strong>~execution_context</strong></span></a> <span class="silver">[destructor]</span>
                    </p>
                </td>
                <td>
                    <p>
                        Destructor.
                    </p>
                </td>
            </tr>
            </tbody>
        </table>
    </div>
    <h5>
        <a name="asio.reference.execution_context.h2"></a>
        <span><a name="asio.reference.execution_context.protected_member_functions"></a></span><a class="link"
                                                                                                  href="execution_context.html#asio.reference.execution_context.protected_member_functions">Protected
        Member Functions</a>
    </h5>
    <div class="informaltable">
        <table class="table">
            <colgroup>
                <col>
                <col>
            </colgroup>
            <thead>
            <tr>
                <th>
                    <p>
                        Name
                    </p>
                </th>
                <th>
                    <p>
                        Description
                    </p>
                </th>
            </tr>
            </thead>
            <tbody>
            <tr>
                <td>
                    <p>
                        <a class="link" href="execution_context/destroy.html" title="execution_context::destroy"><span
                                class="bold"><strong>destroy</strong></span></a>
                    </p>
                </td>
                <td>
                    <p>
                        Destroys all services in the context.
                    </p>
                </td>
            </tr>
            <tr>
                <td>
                    <p>
                        <a class="link" href="execution_context/shutdown.html" title="execution_context::shutdown"><span
                                class="bold"><strong>shutdown</strong></span></a>
                    </p>
                </td>
                <td>
                    <p>
                        Shuts down all services in the context.
                    </p>
                </td>
            </tr>
            </tbody>
        </table>
    </div>
    <h5>
        <a name="asio.reference.execution_context.h3"></a>
        <span><a name="asio.reference.execution_context.friends"></a></span><a class="link"
                                                                               href="execution_context.html#asio.reference.execution_context.friends">Friends</a>
    </h5>
    <div class="informaltable">
        <table class="table">
            <colgroup>
                <col>
                <col>
            </colgroup>
            <thead>
            <tr>
                <th>
                    <p>
                        Name
                    </p>
                </th>
                <th>
                    <p>
                        Description
                    </p>
                </th>
            </tr>
            </thead>
            <tbody>
            <tr>
                <td>
                    <p>
                        <a class="link" href="execution_context/add_service.html"
                           title="execution_context::add_service"><span class="bold"><strong>add_service</strong></span></a>
                    </p>
                </td>
                <td>
                    <p>
                        (Deprecated: Use make_service().) Add a service object to the execution_context.
                    </p>
                </td>
            </tr>
            <tr>
                <td>
                    <p>
                        <a class="link" href="execution_context/has_service.html"
                           title="execution_context::has_service"><span class="bold"><strong>has_service</strong></span></a>
                    </p>
                </td>
                <td>
                    <p>
                        Determine if an execution_context contains a specified service
                        type.
                    </p>
                </td>
            </tr>
            <tr>
                <td>
                    <p>
                        <a class="link" href="execution_context/make_service.html"
                           title="execution_context::make_service"><span
                                class="bold"><strong>make_service</strong></span></a>
                    </p>
                </td>
                <td>
                    <p>
                        Creates a service object and adds it to the execution_context.
                    </p>
                </td>
            </tr>
            <tr>
                <td>
                    <p>
                        <a class="link" href="execution_context/use_service.html"
                           title="execution_context::use_service"><span class="bold"><strong>use_service</strong></span></a>
                    </p>
                </td>
                <td>
                    <p>
                        Obtain the service object corresponding to the given type.
                    </p>
                </td>
            </tr>
            </tbody>
        </table>
    </div>
    <p>
        An execution context represents a place where function objects will be executed.
        An <code class="computeroutput"><span class="identifier">io_context</span></code> is an example
        of an execution context.
    </p>
    <h5>
        <a name="asio.reference.execution_context.h4"></a>
        <span><a name="asio.reference.execution_context.the_execution_context_class_and_services"></a></span><a
            class="link"
            href="execution_context.html#asio.reference.execution_context.the_execution_context_class_and_services">The
        execution_context class and services</a>
    </h5>
    <p>
        Class <a class="link" href="execution_context.html" title="execution_context"><code class="computeroutput"><span
            class="identifier">execution_context</span></code></a>
        implements an extensible, type-safe, polymorphic set of services, indexed
        by service type.
    </p>
    <p>
        Services exist to manage the resources that are shared across an execution
        context. For example, timers may be implemented in terms of a single timer
        queue, and this queue would be stored in a service.
    </p>
    <p>
        Access to the services of an <a class="link" href="execution_context.html" title="execution_context"><code
            class="computeroutput"><span class="identifier">execution_context</span></code></a> is via three function
        templates, <code class="computeroutput"><span class="identifier">use_service</span><span
            class="special">()</span></code>,
        <code class="computeroutput"><span class="identifier">add_service</span><span class="special">()</span></code>
        and <code class="computeroutput"><span class="identifier">has_service</span><span
            class="special">()</span></code>.
    </p>
    <p>
        In a call to <code class="computeroutput"><span class="identifier">use_service</span><span
            class="special">&lt;</span><span class="identifier">Service</span><span class="special">&gt;()</span></code>,
        the type argument chooses a service,
        making available all members of the named type. If <code class="computeroutput"><span
            class="identifier">Service</span></code>
        is not present in an <a class="link" href="execution_context.html" title="execution_context"><code
            class="computeroutput"><span class="identifier">execution_context</span></code></a>, an object of
        type <code class="computeroutput"><span class="identifier">Service</span></code> is created and
        added to the <a class="link" href="execution_context.html" title="execution_context"><code
            class="computeroutput"><span class="identifier">execution_context</span></code></a>. A C++ program
        can check if an <a class="link" href="execution_context.html" title="execution_context"><code
            class="computeroutput"><span class="identifier">execution_context</span></code></a> implements a particular
        service with the function template <code class="computeroutput"><span class="identifier">has_service</span><span
            class="special">&lt;</span><span class="identifier">Service</span><span class="special">&gt;()</span></code>.
    </p>
    <p>
        Service objects may be explicitly added to an <a class="link" href="execution_context.html"
                                                         title="execution_context"><code class="computeroutput"><span
            class="identifier">execution_context</span></code></a> using the function
        template <code class="computeroutput"><span class="identifier">add_service</span><span
            class="special">&lt;</span><span class="identifier">Service</span><span class="special">&gt;()</span></code>.
        If the <code class="computeroutput"><span class="identifier">Service</span></code> is already
        present, the <a class="link" href="service_already_exists.html" title="service_already_exists"><code
            class="computeroutput"><span class="identifier">service_already_exists</span></code></a> exception
        is thrown. If the owner of the service is not the same object as the <a class="link"
                                                                                href="execution_context.html"
                                                                                title="execution_context"><code
            class="computeroutput"><span class="identifier">execution_context</span></code></a>
        parameter, the <a class="link" href="invalid_service_owner.html" title="invalid_service_owner"><code
            class="computeroutput"><span class="identifier">invalid_service_owner</span></code></a> exception
        is thrown.
    </p>
    <p>
        Once a service reference is obtained from an <a class="link" href="execution_context.html"
                                                        title="execution_context"><code class="computeroutput"><span
            class="identifier">execution_context</span></code></a> object by calling
        <code class="computeroutput"><span class="identifier">use_service</span><span class="special">()</span></code>,
        that reference remains usable as long as the owning <a class="link" href="execution_context.html"
                                                               title="execution_context"><code
            class="computeroutput"><span class="identifier">execution_context</span></code></a> object exists.
    </p>
    <p>
        All service implementations have <a class="link" href="execution_context__service.html"
                                            title="execution_context::service"><code class="computeroutput"><span
            class="identifier">execution_context</span><span class="special">::</span><span
            class="identifier">service</span></code></a> as a public base class.
        Custom services may be implemented by deriving from this class and then added
        to an <a class="link" href="execution_context.html" title="execution_context"><code class="computeroutput"><span
            class="identifier">execution_context</span></code></a>
        using the facilities described above.
    </p>
    <h5>
        <a name="asio.reference.execution_context.h5"></a>
        <span><a name="asio.reference.execution_context.the_execution_context_as_a_base_class"></a></span><a
            class="link"
            href="execution_context.html#asio.reference.execution_context.the_execution_context_as_a_base_class">The
        execution_context as a base class</a>
    </h5>
    <p>
        Class <a class="link" href="execution_context.html" title="execution_context"><code class="computeroutput"><span
            class="identifier">execution_context</span></code></a>
        may be used only as a base class for concrete execution context types. The
        <code class="computeroutput"><span class="identifier">io_context</span></code> is an example
        of such a derived type.
    </p>
    <p>
        On destruction, a class that is derived from <a class="link" href="execution_context.html"
                                                        title="execution_context"><code class="computeroutput"><span
            class="identifier">execution_context</span></code></a> must perform
        <code class="computeroutput"><span class="identifier">execution_context</span><span
                class="special">::</span><span class="identifier">shutdown</span><span class="special">()</span></code>
        followed by <code class="computeroutput"><span class="identifier">execution_context</span><span class="special">::</span><span
            class="identifier">destroy</span><span class="special">()</span></code>.
    </p>
    <p>
        This destruction sequence permits programs to simplify their resource management
        by using <code class="computeroutput"><span class="identifier">shared_ptr</span><span
            class="special">&lt;&gt;</span></code>.
        Where an object's lifetime is tied to the lifetime of a connection (or some
        other sequence of asynchronous operations), a <code class="computeroutput"><span
            class="identifier">shared_ptr</span></code>
        to the object would be bound into the handlers for all asynchronous operations
        associated with it. This works as follows:
    </p>
    <div class="itemizedlist">
        <ul class="itemizedlist" type="disc">
            <li class="listitem">
                When a single connection ends, all associated asynchronous operations
                complete. The corresponding handler objects are destroyed, and all <code class="computeroutput"><span
                    class="identifier">shared_ptr</span></code> references to the objects
                are destroyed.
            </li>
            <li class="listitem">
                To shut down the whole program, the <a class="link" href="io_context.html" title="io_context"><code
                    class="computeroutput"><span class="identifier">io_context</span></code></a> function stop() is
                called to terminate any run() calls as soon as possible. The <a class="link" href="io_context.html"
                                                                                title="io_context"><code
                    class="computeroutput"><span class="identifier">io_context</span></code></a> destructor calls
                <code class="computeroutput"><span class="identifier">shutdown</span><span
                        class="special">()</span></code>
                and <code class="computeroutput"><span class="identifier">destroy</span><span class="special">()</span></code>
                to destroy all pending handlers, causing all <code class="computeroutput"><span class="identifier">shared_ptr</span></code>
                references to all connection objects to be destroyed.
            </li>
        </ul>
    </div>
    <h5>
        <a name="asio.reference.execution_context.h6"></a>
        <span><a name="asio.reference.execution_context.requirements"></a></span><a class="link"
                                                                                    href="execution_context.html#asio.reference.execution_context.requirements">Requirements</a>
    </h5>
    <p>
        <span class="emphasis"><em>Header: </em></span><code class="literal">asio/execution_context.hpp</code>
    </p>
    <p>
        <span class="emphasis"><em>Convenience header: </em></span><code class="literal">asio/execution.hpp</code>
    </p>
</div>
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        <td align="left"></td>
        <td align="right">
            <div class="copyright-footer">Copyright © 2003-2022 Christopher M. Kohlhoff<p>
                Distributed under the Boost Software License, Version 1.0. (See accompanying
                file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
            </p>
            </div>
        </td>
    </tr>
</table>
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